| Abstract: | The spread of pathological α-synuclein (α-syn) is a crucial event in the progression of Parkinson’s disease (PD). Cell surface receptors such as lymphocyte activation gene 3 (LAG3) and amyloid precursor-like protein 1 (APLP1) can preferentially bind α-syn in the amyloid over monomeric state to initiate cell-to-cell transmission. However, the molecular mechanism underlying this selective binding is unknown. Here, we perform an array of biophysical experiments and reveal that LAG3 D1 and APLP1 E1 domains commonly use an alkaline surface to bind the acidic C terminus, especially residues 118 to 140, of α-syn. The formation of amyloid fibrils not only can disrupt the intramolecular interactions between the C terminus and the amyloid-forming core of α-syn but can also condense the C terminus on fibril surface, which remarkably increase the binding affinity of α-syn to the receptors. Based on this mechanism, we find that phosphorylation at serine 129 (pS129), a hallmark modification of pathological α-syn, can further enhance the interaction between α-syn fibrils and the receptors. This finding is further confirmed by the higher efficiency of pS129 fibrils in cellular internalization, seeding, and inducing PD-like α-syn pathology in transgenic mice. Our work illuminates the mechanistic understanding on the spread of pathological α-syn and provides structural information for therapeutic targeting on the interaction of α-syn fibrils and receptors as a potential treatment for PD.Aggregation and the spread of amyloid proteins, such as α-synuclein (α-syn), amyloid-β, Tau, and TDP43, are critical events in the pathogenesis of neurodegenerative disorders, including Parkinson''s disease (PD), Alzheimer’s disease, and amyotrophic lateral sclerosis, respectively (1, 2). As the hallmark of PD and other α-synucleinopathies, α-syn aggregation spreads in a prion-like progressive and stepwise manner both within the brain and from other organs to the brain during disease progression (3–7). Pathological α-syn aggregation can template monomeric α-syn to aggregate and participate in disease pathogenesis. Pathological α-syn inclusion can spread in the grafted neurons of PD patients (4, 8). Brain extracts from patients with multiple system atrophy can transmit neurodegeneration to genetically engineered mice (9). A single administration of α-syn preformed fibrils (PFFs) in mouse brains can recapitulate the pathological phenotypes of α-synucleinopathies (10–13).Selected cell surface proteins, such as lymphocyte activation gene 3 (LAG3) and amyloid precursor-like protein 1 (APLP1), have been found to serve as receptors for α-syn PFF internalization and transmission (10, 14, 15). Intriguingly, these receptors preferentially recognize α-syn PFFs rather than the monomer (10). The α-syn monomer is intrinsically disordered and forms α-helical conformation upon membrane binding as involved in synaptic vesicle trafficking (16–19). Cryogenic electron microscopic (cryo-EM) structures of full-length α-syn amyloid fibrils show that the central region of α-syn, approximately covering residues 37 to 99, is involved in the formation of a cross-β fibril core (termed as FC region), while the remaining N and C termini remain flexible (20–24). Despite the recent successes in the structural determination of α-syn amyloid fibrils, considerable challenges remain in linking the structural information to α-syn pathology. The structural basis underlying α-syn transmission, specifically the interplay between α-syn PFFs and receptors, is unknown. It also remains unclear how receptors, for example, LAG3 and APLP1, selectively recognize α-syn PFFs over monomers, nor do we know the role of posttranslational modification of α-syn in this process.In this work, we combined multiple biophysical, cellular, and in vivo approaches to reveal the structural basis underlying the receptor binding of α-syn amyloid fibrils during cell-to-cell transmission. We found that the D1 domain of LAG3 utilizes a positively charged surface to capture the acidic C terminus of α-syn, which is exposed and concentrated on the surface of α-syn fibrils. In contrast, α-syn monomers adopt a self-shielded conformation to impede the exposure of the C terminus. Phosphorylation at serine 129 (pS129) of α-syn, a pathological biomarker in PD (25–27), significantly enhances the binding of α-syn PFFs to LAG3 and APLP1 and promotes the cell-to-cell transmission in vitro and in vivo. Our work provides the structural basis for the receptor-mediated neuronal internalization and transmission of α-syn fibrils and suggests that the C terminus, specifically residues 118 to 140, is a pathological epitope of α-syn for receptor binding and thus may serve as a promising target for the therapeutic drug development to block PD progression. |
